Documentation parsing: recognize \relates, \related, \relatesonly, \relatedonly
so that -Wdocumentation-unknown-command does not warn on these commands.
Fixes PR16092.


git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@184676 91177308-0d34-0410-b5e6-96231b3b80d8